In a crystalline silicon substrate used for a solar cell substrate, the surface of the substrate is made to have a fine pyramidal texture in order to improve the power generation efficiency by increasing the optical path length of incident light. Such a substrate can be obtained by etching (texture etching), a surface of a silicon substrate with an etching solution which may be, for example, a mixture solution of an alkaline solution containing 0.05 to 2 mol/L of NaOH or KOH, and a surfactant whose primary ingredient is caprylic acid or lauric acid of 0.01 mol/L or higher (Patent Document 1). The etching solution needs to be highly alkaline (high pH) in order to raise the etching rate of silicon, and contains an organic substance such as caprylic acid or lauric acid so as to form a texture on the silicon surface. Such an organic substance may be 4-propyl benzoate (Patent Document 2) or isopropyl alcohol (IPA) (Patent Document 3).
As etching processes are repeatedly performed with an etching solution, the pH of the solution drops to a lower value, and concurrently the concentration of SiO2 (silicic acid) in the etching solution and that of dopants such as phosphorus (P) and boron (B) which had been doped into silicon rise. The concentration of the organic additive such as caprylic acid also falls. Consequently, the etching rate decreases and the formation of a fine texture surface becomes difficult, and the etching solution needs to be replaced periodically.
In the apparatus for treating an etching solution described in Patent Document 3, a collecting tank to separate dopants such as phosphorus and boron dissolved in the etching solution from the etching solution by means of adsorption, precipitation or electrical collection is installed, end the etching solution is reused. However, the frequency at which replacement is conducted cannot be sufficiently reduced, and, because metal salts, porous substances and the like come into contact with the etching solution, impurities derived from the metal salts and porous substances may commingle with each other.